In an engine device of an automobile or the like, in order to improve engine output, there has been performed “supercharging” for driving an exhaust turbine by exhaust gas discharged from the engine, thereby coaxially driving a compressor disposed in an intake passage, and compressing intake gas to be supplied to the engine.
In the supercharging by the turbocharger, a response delay at the time of engine low speed rotation called turbo lag becomes a problem. As a technique of complementing the response delay due to the turbo lag, there has been known a two-stage supercharging system provided with an electric supercharger driven by the turbocharger and by an electric motor. Further, there has also been known an electrically assisted turbo that has an electric motor attached to the turbocharger, and eliminates turbo lag by driving the compressor by operating the electric motor during a low speed rotation (Patent Document 1).
In the following specification, a supercharger configured to be able to drive the compressor by the electric motor will be referred to as a “supercharger with an electric motor”. The supercharger with an electric motor also includes an “electric supercharger” that is not provided with an exhaust turbine and has the compressor driven by only the electric motor, and an “electrically assisted turbo” that has the compressor driven by driving the exhaust turbine in addition to the electric motor.